GSK-2634673F

Pancreatic ductal adenocarcinoma (PDAC) is known to be one of the most lethal cancers. Since the majority of patients are diagnosed at an advanced stage, development of a detection method for PDAC at an earlier stage of disease progression is strongly desirable. Integrin α_Vβ6 is a promising target for early PDAC detection because its expression increases during precancerous changes. The present study aimed to develop an imaging probe for positron emission tomography (PET) which targets α_Vβ6 integrin-positive PDAC. We selected A20FMDV2 peptide, which binds specifically to αvβ6 integrin, as a probe scaffold, and ⁶⁸Ga as a radioisotope. A20FMDV2 peptide has not been previously labeled with ⁶⁸Ga. A cysteine residue was introduced to the N-terminus of the probe at a site-specific conjugation of maleimide-NOTA (mal-NOTA) chelate. Different numbers of glycine residues were also introduced between cysteine and the A20FMDV2 sequence as a spacer in order to reduce the steric hindrance of the mal-NOTA on the binding probe to αVβ6 integrin. In vitro, the competitive binding assay revealed that probes containing a 6-glycine linker ([^natGa]CG6 and [^natGa]Ac-CG6) showed high affinity to αVβ6 integrin. Both probes could be labeled by ^67/68Ga with high radiochemical yield (>50%) and purity (>98%). On biodistribution analysis, [⁶⁷Ga]Ac-CG6 showed higher tumor accumulation, faster blood clearance, and lower accumulation in the surrounding organs of pancreas than did [⁶⁷Ga]CG6. The α_Vβ6 integrin-positive xenografts were clearly visualized by PET imaging with [⁶⁸Ga]Ac-CG6. The intratumoral distribution of [⁶⁸Ga]Ac-CG6 coincided with the α_Vβ6 integrin-positive regions detected by immunohistochemistry. Thus, [⁶⁸Ga]Ac-CG6 is a useful peptide probe for the imaging of α_Vβ6 integrin in PDAC.

The alpha-v beta-6 (αvβ6) integrin has been identified as playing a key role in the activation of transforming growth factor-β (TGFβ) that is hypothesised to be pivotal in the development of cancer and fibrotic diseases. Therefore, the αvβ6 integrin is an attractive therapeutic target for these debilitating diseases and a drug discovery programme to identify small molecule αvβ6 selective arginyl-glycinyl-aspartic acid (RGD)-mimetics was initiated within GlaxoSmithKline. The primary aim of this study was to pharmacologically characterise the binding to αvβ6 of a novel clinical candidate, compound 1, using a radiolabelled form. Radioligand binding studies were completed with [(3)H]compound 1 against the human and mouse soluble protein forms of αvβ6 to determine accurate affinity estimates and binding kinetics. The selectivity of compound 1 for the RGD integrin family was also determined using saturation binding studies (αvβ1, αvβ3, αvβ5, αvβ8, α5β1 and α8β1 integrins) and fibrinogen-induced platelet aggregation (αIIbβ3 integrin). In addition, the relationship between divalent metal cation type and concentration and αvβ6 RGD site binding was also investigated. Compound 1 has been demonstrated to bind with extremely high affinity and selectivity for the αvβ6 integrin and has the potential as a clinical tool and therapeutic for investigating the role of αvβ6 in a range of disease states both pre-clinically and clinically. In addition, this is the first study that has successfully applied radioligand binding to the RGD integrin field to accurately determine the affinity and selectivity profile of a small molecule RGD-mimetic.